Complexes [(P-2)Rh(hfacac)] as model compounds for the fragment [(P-2)Rh] and as highly active catalysts for CO2 hydrogenation: The accessible molecular surface (AMS) model as an approach to quantifying the intrinsic steric properties of chelating ligands in homogeneous catalysis

The complexes [(P-2)Rh(hfacac)] 1 [P-2 = R2P-(X)-PR2] are introduced as model compounds for the investigation of the intrinsic steric properties of the [(P-2)Rh] fragment. The ligand exchange processes that occur during the syntheses of 1 from [(cod)Rh(hfacac)] and the appropriate chelating diphosphanes 3 were studied by variable-temperature multinuclear NMR spectroscopy. The molecular structures of eight examples of 1 with systematic structural variations in 3 were determined by X-ray crystallography. The steric repulsion of the PR2 groups within the chelating fragment was found to significantly influence the coordination geometry of [(P-2)Rh], depending on the nature and length of the backbone (X). A linear correlation between the P-Rh-P angles in the solid state and the Rh-103 chemical shifts reveals a similar geometric situation in solution. A unique molecular modeling approach was developed to define the accessible molecular surface (AMS) of the rhodium center within the flexible [(P-2)Rh] fragment. The potential of this model for application in homogeneous catalysis was exemplified by the use of 1 as catalysts in a test reaction, the hydrogenation of CO2 to formic acid. Complexes 1 were found to be the most active catalyst precursors for this process in organic solvents known to date.